use only 3 types of hashtable sizes

[PSeitz]

• cleanup
• switch to use only 3 kinds of hashtable

[codecov]

Codecov Report

Merging #77 (d948f50) into main (10627fe) will increase coverage by 0.96%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #77      +/-   ##
==========================================
+ Coverage   88.75%   89.71%   +0.96%     
==========================================
  Files          11       11              
  Lines        2214     2149      -65     
==========================================
- Hits         1965     1928      -37     
+ Misses        249      221      -28     

Impacted Files	Coverage Δ
src/block/compress.rs	99.83% <100.00%> (+0.81%)	⬆️
src/block/hashtable.rs	80.00% <100.00%> (+11.40%)	⬆️
src/frame/compress.rs	83.33% <100.00%> (ø)

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